Banner and click exchange programs. You can just serach Google for that. Generally, you are requird to put an ad on your website. Then you get credits each time somebody clicks an ad on your site, and then people will click your ad on other sites, which will use credits. Seems like a nice idea, and there's no risk involved since it's free.
When I had a content site, I used the following banner exchange:
http://www.bannerxnetwork.com
They're decent. But you get what you pay for, so no complaints. They have a forum where you can discuss their site.
Hope this helps.